An amplifier driven speaker coupled to a magnetic
pick-up coil can produce a feedback squeal. This squeal can be
blocked by a steel plate. Its frequency can be changed by using
aluminum foil.

Material

An amplifier and speaker (Radio
Shack)

A pick up coil (Sold as a "Telephone pick-up"
at Radio Shack)

Aluminum foil

steel plate at least 1/16 inch
thick.

Assembly

Plug the pick up coil into the input of the
amplified speaker.

Basic exploration

To Do and Notice

Turn on the amplifier and turn it to maximum
amplification.
Notice that you can hear the sound of static.
Move the pick-up coil toward the speaker.
Notice the annoying squeal.
Turn down the gain of the amplifier, notice that you must move the
pick-up closer to the amplifier to produce feedback.

Whats Going On?

At high gain you hear "static" as the speaker
amplifies noise. The noise is produced when room temperature makes
electrons move randomly back and forth in the amplifier's input. The
amplifier amplifies these random electrical currents and sends them
to a speaker. In the coils of wire in the speaker the currents
produce changing magnetic fields as well as sound.
The pick-up coil detects the changing magnetic field of the noise and
feeds them back to the amplifier, which is why we call this process
feedback. The amplifier then amplifies them again and sends them to
the speaker once again. Each time around the signal gets louder.
The amplified speaker does not amplify all frequencies by an equal
amount. There is a frequency at which the gain will be maximum. This
is the frequency of the squeal of the feedback. This frequency is
determined by the electronics of the amplifier, and by the
capacitance of the circuit amnd the inductance of the coils.

If you bring a piece of iron near the coil you
change its inductance. We'll explore the efects of this change in the
next exploration.

Advanced exploration

To Do and Notice

Place the pick-up at a distance that will produce
the squeal of feedback. Insert the sheet of steel between the
amplifier and the pick-up coil. The feedback will stop.

Whats Going On?

The magnetic fields of the speaker align the
ferromagnetic iron atoms in the steel so that the magnetic fields on
the other side of the sheet of steel are diminished or canceled. This
reduces the feedback and stops the squeal.

To Do and Notice

Remove the steel and create a squeal once
again.
Insert a sheet of aluminum foil between the speaker and the pick-up
coil.
Notice that the pitch of the sound increases.
Fold the aluminum foil over so that it is 4 sheets thick and insert
it again. Notice that the pitch increases even more.
Cut slits in the 4 sheet thick aluminum foil and insert it again.
Notice that the slit aluminum foil does not change the pitch very
much.
Punch holes in 4 sheet thick aluminum foil using a hole punch.
Notice that the holed aluminum foil does increase the pitch of the
squeal.

Aluminum foil squares, one with slices, one with
holes

What's Going On?

The changing magnetic fields create eddy currents
in the aluminum foil which create magnetic fields in turn. This
changes the inductance of the pick-up coil and raises the frequency
of the resonant peak.
Thickening the aluminum foil increases the eddy currents.
Slitting the aluminum foil reduces the eddy currents.
Punching holes in the aluminum foil does not impede the eddy
currents.

To Do and Notice

Insert a piece of iron into the space between the
speaker and the coil. Listen to the change in pitch. There are two
competing effects: iron is a metal and so conducts eddy currents and
iron is ferromagnetic. The eddy currents increase the frequency of
the feedback while the ferromagnetism decreases the frequency. So
iron can either increase or decrease the frequency of the oscillation
depending on it conductivity.

So What?

Metal Detectors

The amplifier and coil make a simple metal
detector. You can hear when metal comes close to the coil. Real metal
detectors also use coil. They transmit magnetic fields into their
vicinity and then receive these fields. The fields are modified by
the presence of ferrous and nonferrous metals.

Automobile detectors

Many Traffic lights employ coils in the pavement
to sense the presence of automobiles. The metal of the automobile
changes the resonant frequency of the coil. The change in resonant
frequency is sensed and triggers the signal. The iron in the car acts
more by its eddy currents than by its ferromagnetic nature so the
resonant frequency of the circuit increases in the metal detecting
circuot when a car drives over the coils.